THE MEASUREMENT OF LABILE PHOSPHORUS BY THE ISOTOPIC DILUTION AND ANION RESIN METHODS

Abstract

We measured labile P in a group of non-calcareous agricultural soils by isotopic exchange at four different levels of carrier P and by an anion resin extraction. The soils were from the Ultisol, Alfisol, and Mollisol orders and had been evaluated as low, medium, and high in available P on the basis of soil test calibration studies. Differences among the labile P values determined at the four carrier levels were found in soils low in P, but not in soils having medium and high P contents. Greatest differences were found on the low-P Ultisols and were attributed to 32P fixation. The resin P and labile P measurements were comparable on the medium- and high-P Alfisols and Mollisols, indicating that, when difficulties are not encountered with the isotopic technique, the isotopic and resin extraction procedures will measure the same or nearly the same phosphate pool. On the low-P Alfisols and Mollisols, labile P measurements made at the low (4 x 10−6M P) and medium (2 x 10−5M P) carrier P levels most closely approximated the resin P results. A carrier P level within this range is recommended for use on low-to medium-P-fixing soils to minimize problems with the isotopic exchange procedure. On the high-P-fixing Ultisols comparison of isotopically exchangeable labile P with the resin P values showed that the similarity of the results depended on the amorphous Fe and P content of the soils. The results demonstrate that on soils with a high P-fixation capacity, the isotopic exchange technique should not be used to measure labile P, but that labile P can be measured by an anion resin extraction on these, as well as on low-P-fixing soils.